This invention concerns improvements in the processing of video image signals to achieve a digital video effect known as `decay`. In this effect a video image, or a part of a video image is caused to disappear with time in a variety of ways. This is achieved by processing the so-called key signal associated with a video image in order to vary the magnitude of the key signal associated with individual pixels or groups of pixels of the video image. For example the magnitude of the key signal can be reduced identically at every point of the image, to give a gradual decay, the key signal associated with groups of pixels may be reduced to form shaped holes in the image in a given sequence until the image disappears, or the key signal associated with individual pixels or groups of pixels may be reduced in a random manner to give a so-called random decay effect.
The processing of the key signal in the required manner is achieved by causing the key signal to be recirculated through a frame store via a path including a decay processor for processing the key signal, during the period of each frame of the video signal to be reproduced. Thus for the reproduction of each frame of the video image the corresponding key signal is read from the frame store and then returned to the frame store to be read again for the reproduction of the next frame. Since the key signal read from the frame store is modified by the decay processor before being returned to the store, a controlled decay is introduced into each consecutive frame of the video image in order to achieve the desired visual effect.
A drawback of the known arrangement, however, is that the range of decay effects that can be obtained is limited. It is impossible, for example, to make a decaying image appear to sparkle or to make it look as though it is reflecting light from random directions, like glitter.